Hi Joshua, just ask your sister to contact MTN support, they will ask her some security questions on the details of the line, if she got them correct, they will reset the transfer pin for her.
You can contact MTN support via any of these channels:
1. MyMTN App
2. Facebook: @MTNLoaded
3. Instagram: @MTNNG
4. Live Chat: http://bit.ly/2OC5bHz
5. Email: customercareng@mtn.com
6 .WhatsApp: 09033000001